Results for specimens of the refractory lining sampled from the bottom of an aluminum electrolysis cell with a service life of 3.5 years examined by methods of petrography, chemical analysis, and electron probe x-ray microanalysis are reported. The main products of conversion are sodium aluminosilicates, sodium silicates, a glassy phase of variable composition, oxyfluoride glasses, and eutectics. Some of the specimens analyzed are high in b-alumina (20 -50%). Fluorides are represented by NaF (about 7%), cryolite Na 3 AlF 6 (2 -5%), malladrite Na 2 SiF 6 (2%), and NaF × MgF 2 (1 -2%), and the metallic phase, by aluminum (2 -7%) and ferrosilicon (3 -10%). The apparent density of the used refractory material is 2.5 -2.62 g/cm 3 .Data have been reported [1 -4] on the use of a-alumina as a component of the cathode lining for aluminum electrolysis cells, on fluoride salts as impregnators for the lining, and on the phase transformations in a-Al 2 O 3 involved. In [5,6], results of an inspection of a chamotte refractory specimen sampled from the bottom lining of the base plate of a dismantled aluminum electrolysis cell were given.Here we report results of a study of specimens sampled from different parts of the chamotte refractory lining of the bottom of an aluminum electrolysis cell that had been in service for 3.5 years. The lining of the bottom of the electrolysis cell was composed of a heat-insulating vermiculite layer 100 mm thick, a layer of dry barrier mixture (DBM) 110 mm thick, and a refractory layer of ShA-5-grade chamotte refractory 65 mm thick.Methods of petrography, chemical analysis, and electron probe x-ray microanalysis (EPXMA) were used in our study. The specimens were observed in reflected and transmitted light under MBI-6 and MIN-8 microscopes at magnifications of´90 -400, and in reflected light at magnifications of 950 -1800 using an immersion oil. The EPXMA was carried out on an MS-46 Cameca microprobe analyzer.A total of 50 specimens were sampled from the disused refractory lining. The sampling scheme is shown in Fig. 1. Chemical analysis results are summarized in Table 1.In this communication, results of a comparative analysis of specimens of intact (pre-service) ShA-5 chamotte refractory and six specimens of post-service chamotte 6-3m; 24-1m, 33-1t, 34-3m, 38-2m, and 40-1m are given.The commercial ShA-5 chamotte refractory contained (according to manufacturer's specifications) 37 -41% Al 2 O 3 , with an open porosity of 22.4 -24%. The apparent density of post-service ShA was 2.50 -2.62 g/cm 3 (specimens 6-3m and 38-2m).The inspected specimens of refractory layer were present in a range of colors -green, gray, yellow-brown, occasionally milky in appearance, hard and stony to touch, in places exhibiting a zonal pattern. Specimen 33-1t was powdery, of gray-brown color. The surface of solid, powder-like, and milky specimens, when tested with phenolphthalein, gave an alkaline reaction. The characteristic crimson color took different times to develop from specimen to specimen -either instantly or in a ...

The heat insulating layer of an aluminum electrolyzer is shown to become gradually impregnated with sodium and fluoride compounds. Vermiculite, while subjected to deep dehydration, enters into reaction with electrolysis products, which resulted in damage to the refractory chamotte layer. The saturation with gaseous compounds proceeds along the cleavage plane of vermiculite. Sodium tetrafluoraluminate is shown to form under the electrolyzer's service conditions. Alongside the pure fluorides, complex fluorides of variable composition have been identified. Fluorides and metallic aluminum have been shown to form by condensation and disproportionation of lower fluorides. Electric corrosion is suggested as a factor affecting the structural integrity of the electrolyzer's casing.A major route towards increasing the service life of aluminum electrolyzers consists in improving the heat insulation of the electrolyzer's casing to provide a reliable protection of the bath from heat losses and to maintain a scull coating built-up on the refractory lining. At present, vermiculite plates have found use as the heat-insulating material in some aluminum electrolyzers. In this paper, we present the results of a study of phase transformations that occurred in the vermiculite heat-insulating coating after 45.6 months of service of the Al electrolyzer. The sampling scheme and chemical analysis methodology can be found in [1]. Sampled specimens labeled 1-1s, 3-1s, 5-2s, 7-3v, 11-2s, 14-3s, 15-1v, and 22-1n were examined [1].Vermiculite of natural occurrence is a gold-colored mineral, of complex and variable composition (Mg, Fe 2+ , Fe 3+ ) 3 [(Al, Si) 4 O 10 ](OH) 2 × 4H 2 O. Based on chemical analysis data, the precursor vermiculite plates had an elemental composition (wt.%) of Mg 9.78, Al 4.23, and Si 19.61. The initial density of vermiculite plate material was 0.4 g/cm 3 . The apparent density of specimens sampled from the post-service heat-insulating layer was 1.623 -1.187 g/cm 3 (specimens 3-1s and 15-1v).Macroscopic specimens sampled from the post-service heat-insulating layer looked like: 1-1s, gray brittle chips with white inclusions or a powder of gray color; 3-1s, brittle chips or a powder of yellow-gray color with vermiculite flakes; 5-2s, a powder of gray color with vermiculite flakes; 7-3v, rather large lumps or a powder of pink color with vermiculite platelets; 11-2s, a powder of red-brown color with little balls and flakes of residual vermiculite; 14-3s, a powder of red color with vermiculite flakes; 15-1v, chips or a powder of white-gray color with flakes of residual vermiculite; and 22-1n, a powder of dark-gray color with vermiculite flakes.The phase composition and quantitative estimates of phase transformation within the heat-insulating layer as determined by petrographic analysis are given in Table 1. With time, the vermiculite plates of the heat-insulating layer became impregnated with sodium and fluoride salts. The plates, originally made up of pressed and partially dewatered vermiculite, gradually transformed in...

It has been established that transformations within the layer of a dry barrier mixture (DBM) are selective in character and are mainly confined to the upper zones of the innermost part (core) of DBM. The DBM layer undergoes a solid-liquid sintering in these zones. The glass phase thus formed prevents the electrolysis products from penetration into the DBM layer. Modifier admixtures -titanium and iron oxides, products of electrolysis and degradation of the refractory layer AlF, SiF 4 , and sodium oxide -decrease the viscosity of aluminosilicate melt. The grains of a disthene-sillimanite concentrate undergo cracking along the cleavage plane. Imperfections of the crystal lattice promote transformations in the grains of normal electrofused corundum. Oxyfluoride AlOF and an oxyfluoride of variable composition AlOF 1 -x are formed in the cathode lining of the electrolysis cell.In this paper we present the results of a study of the dry barrier mixture (DBM) sampled for analysis from the bottom lining of an aluminum electrolysis cell after 45.6 months of service life. This work is a continuation of our previous studies reported in [1 -3].Originally, the DBM was a mixture of granular disthene-sillimanite concentrate (GDSC) and normal electrofused corundum (NEC) of fraction 20 mm. According to TU U 14-10-017-98 Specifications, GDSC contains, wt.%: Al 2 O 3 , not less than 57; iron oxides, not higher than 0.8%; CaO, not higher than 0.4; TiO 2 , not higher than 2.5. GDSC consists mainly of aluminum silicates -disthene Al 2 [SiO 4 ]O and sillimanite Al[AlSiO 5 ]. Disthene and sillimanite are the same in chemical composition, but they differ structurally. Both minerals are eutomous. The strength of disthene is emphasized by its name (di + Greek sthene force) [4]. So, its hardness in the cleavage plane lengthwise is 4.5; across, it is 6. The hardness of sillimanite (named after the American chemist and geologist Benjamin Silliman, 1779 -1864) is 6.5 -7. Occasionally, quartz grits and glass with N = 1.54 are found among other impurities in GDSC.The NEC micropowder consists mainly of a-Al 2 O 3 . The NEC chemical composition is, wt.%: Al 2 O 3 , not less than 94; Fe 2 O 3 , not higher than 0.6; TiO 2 , not higher than 2.8; SiO 2 , not higher than 1.3; CaO, not higher than 0.7. A specific feature of NEC is the occurrence of Ti 3+ in it in the form of a solid solution [5,6]. Oxides of titanium and iron were also found in the basal jointing of a crystalline electrocorundum. These impurities increase the degree of the a-Al 2 O 3 crystal lattice imperfection. In the powdered NEC, rutile TiO 2 occurs as an individual impurity phase. The mentioned features of constituents GDSC and NEC cannot leave unaffected the DBM performance characteristics.Specimens 9-2n, 12-3s, 18-3s, 20-1v, 21-1n, 26-2v, 28-1v, 29-1n, 39-2s, 41-1v, and 44-1n were examined petrographically. The sampling scheme and chemical analysis of the DBM post-service specimens were as described in [1]. The density of DBM post-service specimens determined by hydrostatic weigh...

Specimens of post-service ShA-5-grade chamotte refractory are analyzed for total elemental aluminum and silicon. Products of the electrolysis of cryolitic melt Na (gas), AlF, Al 2 O, and, possibly, Na 2 F are shown to diffuse through pores of the carbon refractory bottom lining blocks of an aluminum electrolysis cell. The lower compounds and Na(gas) as they reach the refractory layer enter into a reaction with the refractory material. The Gibbs free energy of hypothetic reactions in the Al -Si -O -F -Na system involving a gas phase has been calculated. It is inferred that the performance characteristics of the ShA-5 refractory fail to meet standard operational requirements.In a previous communication [1], the changes in mineral and phase composition of ShA-5-grade chamotte refractory specimens sampled from the bottom lining of a aluminum electrolysis cell in service for 45.6 months have been reported. In this paper, we focused attention on a thermodynamic analysis of the processes involved in the wear of the refractory lining of the electrolysis cell bottom.The results of a post-service analysis of the ShA-5 refractory have indicated an increase in total aluminum in some test specimens (24-1m and 33-1m) and a decrease in total silicon, especially in specimens 6-3m, 34-3m, and 40-1m. The increase in total aluminum defies explanation as being due to the simple penetration of metallic aluminum into the refractory layer. The mode of occurrence and dispersion of the penetrated metal suggest that the infiltration of aluminum is effected via gas phase; that is, the electrolysis products AlF and Al 2 O 3 penetrate through pores and joints in the carbon bottom lining blocks to undergo disproportionation during condensation from the gas phase by the reactions 3AlF (gas) = 2Al + AlF, 3Al 2 O (gas) = 4Al + g-Al 2 O 3 , 3AlO = Al + g-Al 2 O 3 .